204 



WILLIAM H. HOBBS 



The relationship to each other in position of the hstric surfaces 

 which are produced by drag suggests the pecuUar shearing surfaces 

 which are seen at the snouts of some glaciers, save only that the 

 distribution of forces is essentially reciprocal. It is the upper layers 

 of the glacier which are pushed /orwarrf and override the lower layers 

 held back by friction (Fig. 36). As a consequence, and as already 

 pointed out {ante, p. 181), it is the upper limb of each fold rather 

 than the lower that here becomes attenuated. 



Extended sHdes within folded strata of mountains would appear 

 to be greatly favored by the presence of a weak formation imme- 

 diately above the competent member, this weak formation acting, 

 so to speak, as a lubricant for the slide surface. It is no doubt 



significant that in the Alps the 



Fig. 36. — Slide surfaces in a glacier 

 snout (after T. C. Chamberlin). 



weak Flysch overlies the Helve- 

 tian limestones, for this yielding 

 formation underlies one great 

 shde after another in the series 

 — 'Wildflysch^^ (Figs. 34 and 35) . 

 It may also be of significance 

 that within the same region the 

 formation against which the 

 sKdes rise at the front with 

 the greatest development of 

 listric surfaces is the hard Nagelfluh conglomerate, which has a 

 local development only upon the northern margin of the Alps, and 

 opposite whose areas rise the largest accumulation of rock sHces. 



Examination of numerous profiles which include rock slides 

 indicate pretty clearly that portions of the indriven sHces have 

 sometimes become involved in the complex of wedges at the front 

 of each overlying slice (Fig. 37). Sections of the Belgian coal- 

 field (Fig. 37, e and/) and of Buffalo Mountain' (Fig. 38) may be 

 cited as examples. 



SUMMARY 



The Alps represent the type of Asiatic mountain arcs, and hence 

 in explaining their tectonics deductions from Asiatic studies are 



'Arthur Keith, "Roan Mountain Folio, Tennessee-North Carohna," Folio 151, 

 U.S. Geol. Surv., 1907, p. 9. 



